Evidence for myosin-binding phosphatase in heart myofibrils

Protein phosphatase was partially purified from myofibrils of bovine heart by sequential column chromatographies. The purified protein phosphatase was immunologically identified as a delta isoform of PP1 (PP1delta). The myosin-binding subunit (MBS) of myosin-binding phosphatase (MBP) in smooth muscle was co-purified with PP1delta at each step of the sequential column chromatographies. The immunoprecipitation experiment using the polyclonal antibody to MBS showed that PP1delta associates with MBS in the purified phosphatase. In addition, the myosin-binding assay showed that the purified phosphatase has the characteristics of binding to cardiac myosin. These data strongly suggest that MBP, the holoenzyme composed of PP1delta and MBS, is expressed in heart myofibrils.